1. Field of Invention
The present invention relates to a metal oxide-based thermoelectric conversion material for use in thermoelectric conversion elements.
2. Background Art
In recent years, thermoelectric conversion elements using metal oxides have been proposed as a technique that solves problems of high-temperature durability and toxicity involved in conventional metal compound-based thermoelectric conversion elements, and studies on the thermoelectric conversion elements using metal oxides have made rapid progress. In general, properties of thermoelectric conversion elements are expressed by a few characteristic factors using a Seebeck coefficient α (μV·K−1) that is a thermoelectromotive force per unit temperature difference, an electroconductivity σ (S·cm−1), and a coefficient of thermal conductivity κ (W·m−1·K−1). One of the factors is a thermoelectric power factor represented by α2σ. Further, a figure-of-merit obtained by dividing a thermoelectric power factor by a coefficient of thermal conductivity, i.e., Z (=α2σ/κ), and a dimensionless figure-of-merit ZT obtained by multiplying a figure-of-merit Z by an absolute temperature T are used as performance measures. In general, the larger these values, the better the thermoelectric properties.
The thermoelectric conversion element is generally prepared by combining two types of metals or semiconductors. A combination of a p-type semiconductor in which carriers are holes with an n-type semiconductor in which electrons are carriers is required in order to generate electric power with high efficiency. In existing oxide semiconductors, a high ZT of about 0.7 (NaCO2O4 polycrystal) comparable to that of metal compounds is reported in the p-type. On the other hand, in the n-type, the ZT is up to about 0.3 and is an obstacle to the popularization of thermoelectric conversion devices using metal oxides. At the present time, a demand for the breakthrough of thermoelectric properties of n-type oxide semiconductors has become more and more very strong.
Aluminum (Al) doped zinc oxide (Al—ZnO) is known as an n-type oxide semiconductor that exhibits a high level of thermoelectric properties (PTL 1). In Al—ZnO, however, a very high coefficient of thermal conductivity due to a high debye temperature and a high acoustic phonon speed is a factor that inhibits a further improvement in ZT. In order to lower the coefficient of thermal conductivity of Al—ZnO-based materials, studies have hitherto been made, for example, on formation of solid-solution of Mg or Ni co-doped with Al in ZnO (NPL 1). Further, there is a report about a system that is based on Al—ZnO and is co-doped with La (PTL 2) and a system that is based on Al—ZnO and is co-doped with Ce (PTL 3). In these prior art techniques, the particle diameter of zinc oxide is preferably not more than 200 nm (for example, paragraphs 0006 and 0013 of PTL 2).
ZnO sintered compacts doped with yttrium (Y) have also been studied (PTL 4). Likewise, zinc oxide sintered compacts doped with praseodymium (Pr) have also been studied (NPL 2).